A switch mode power converter is a multi-port network having at least one input port and at least one output port. The power converter includes a switching cell, which transfers electrical energy from the input ports to the output ports. The switching cell includes at least one inductor, and may also include one or more capacitors. The switching cell includes at least one controlled switch, such as a power transistor, and possibly one or more diode rectifiers. The switching cell also has at least one control port that drives the switches by adjusting the switch conduction periods so as to regulate the flow of electrical power through the converter.
Commonly, a switch mode power converter is used as a voltage-regulated power supply. In such a power supply, the output voltage is held substantially constant irrespective of fluctuations in the output current and the input voltage. The quality of such a voltage-regulated power supply is measured by characteristics such as load regulation, line regulation, and transient response. The load regulation of a power supply represents the susceptibility of the dc voltage output of the power supply to fluctuations in load current. The line regulation of a power supply represents the susceptibility of the dc output voltage of the power supply to fluctuations in input voltage. The transient response represents the magnitude of the overshoot (or undershoot) and the settling time for the output of the power supply in response to a step change in line voltage or load current. It is desirable to have a voltage regulated power supply that has good load and line regulation and a fast transient response.
The behavior of voltage-regulated power supplies is dependent upon the current within the inductor of the switching cell, and two modes of operation are distinguished upon this basis. The first mode of operation is referred to as continuous mode. In continuous mode, the first derivative of the inductor current is continuous within each topological state of the circuit. The second mode of operation is referred to as discontinuous mode. In discontinuous mode of operation, the first derivative of the inductor current is discontinuous within one topological state of the circuit. This means that the current in the inductor drops to zero and remains at zero for some finite length of time. In each mode of operation, a particular voltage regulated power supply may have a distinct frequency response with independent compensation requirements.
Switching cells can be broadly divided into two categories: buck derived and boost derived cells. Buck derived cells include the buck, forward push-pull, half-bridge and full bridge topologies. Boost derived cells include the boost, inverting and fly-back topologies. In continuous mode, boost derived topologies exhibit a right half plane zero that greatly complicates stabilization.
Voltage regulated power supplies control the output of the switching cell with some form of negative feedback. Two prior common control schemes are referred to as voltage mode control and current mode control. Each is described in detail below. With voltage mode control, the output voltage of the power converter is sensed by a feedback circuit and is used to control the converter. The feedback circuit includes an error amplifier which compares the output voltage to a fixed reference voltage and generates an error signal. A modulator circuit transforms the error signal into a train of pulses which are fed to the control port of the switching cell. The output of the modulator circuit controls one or more switches in the switching cell so as to regulate the output voltage. In discontinuous mode, the voltage mode control strategy applied to a buck derived topology exhibits a single pole frequency response which typically does not require additional compensation. A small valued capacitor is often used to create a high frequency pole for noise suppression. But, this pole is positioned at a sufficiently high frequency to prevent it from significantly degrading the phase margin below the unity gain frequency. This provides good line and load regulation as well as reasonably fast transient response. However, in continuous mode, the appearance of a pair of poles in the transfer function representing the control loop requires a lead-lag network to provide adequate phase margin to ensure stability. Although this configuration provides good line and load regulation, it has a slow transient response due to the large time constant of the lead-lag network.
Other prior converters use a current mode control strategy which monitors an analog of inductor current as well as output voltage to provide a substantially regulated output voltage. One type of current mode control is referred to as peak-current-commanding control. This control scheme, like voltage mode control, uses an error amplifier which compares the output voltage to a fixed referenced voltage to generate an error signal. This error signal is used to set the peak current permitted to flow in the inductor. A second, or minor loop, measures the inductor current and feeds it back as a second input to a comparator. The comparator compares the peak current sensed in the inductor to the error signal generated by the error amplifier. The comparator controls the switch of the switching cell through an oscillator-controlled flip-flop, so that the switch will be disabled when the peak inductor current reaches the threshold set by the error signal. In continuous mode, this circuit exhibits a single pole response rather than the-two pole response characteristic of voltage mode control. The single pole response characteristic of this circuit simplifies the compensation of the converter. Peak current commanding current mode control also simplifies implementation of current limiting for protection of the power devices contained in the switching circuit. Additionally, the minor loop provides feedforward correction of line voltage variations by responding immediately to any change in line voltage. This provides superior transient response to line voltage fluctuations. However, current mode control also exhibits several difficulties. First, current mode control exhibits open-loop instabilities (subharmonic oscillations) where the duty cycle of the switch exceeds fifty percent. Additionally, it is difficult to properly sense the current in the inductor without either employing costly current transformers or risking significant noise pick-up from small valued current sense resistors.
Another prior scheme for controlling a switch mode power converter is referred to as direct-summing current mode control. In this configuration, the error amplifier is replaced with a direct feedback to one input port of a summing comparator. The summing comparator has two pairs of differential inputs. The output of the comparator switches when the sum of the differential voltages across the input ports is equal to zero. The first differential input is connected so as to receive the difference between the output voltage and a fixed reference voltage. The inductor current is sensed differentially across a small resistor and is fed back to the second differential input to the summing comparator. Without the error amplifier, the gain of the circuit is substantially reduced. If the magnitude of the current sense signal is kept small, the circuit gain can be increased to practical levels without the use of an error amplifier. Eliminating the error amplifier eliminates the excess phase shift associated with this component, somewhat simplifying compensation. The circuit is also less complex and the current consumption associated with the error amplifier is eliminated. Direct summing current mode control has inherently poor load regulation because the gain of the loop is limited by how small the current sense voltage can be made. If this voltage is too small, the circuit will be unduly noise sensitive.